MB-8066 Maintain the high priority vbucket counter per shard

As we have a separate flusher per shard, we need to maintain
the high priority vbucket counter per shard.

Change-Id: I03a626006fb3f9b285e84d1927e9bd44ead70f02
Reviewed-on: http://review.couchbase.org/26448
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>